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JPA Grouping
Posted on: October 4, 2006 at 12:00 AM

In this section, you will learn about the JPA Grouping and how to use it in your JPA application

JPA Grouping

In this section, you will learn about the JPA Grouping and how to use it in your JPA application. The GROUP BY clause is used with SELECT statement to retrieve multiple records in a group. The selected result is displayed by one or more columns.

JPA Grouping:

Query query=em.createQuery("SELECT st FROM Student st group by st.sname");

For JPA Grouping application, you need the following artifacts:

Database Table: student
Model Class: Student.java
Main Class: JPAGrouping.java

Database Table: student

CREATE TABLE `student` (
`id` int(11) NOT NULL auto_increment,
`sname` varchar(100) NOT NULL,
`sroll` int(11) NOT NULL,
`scourse` varchar(10) NOT NULL,
PRIMARY KEY (`id`)
)

Model Class: Student.java

/**
*
*/
package roseindia;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Table;

/**
* @author Administrator
*
*/
@Entity
@Table(name="student")

public class Student {

@Id
@GeneratedValue
private int id;

/**
* @return the id
*/
public int getId() {
return id;
}

/**
* @param id the id to set
*/
public void setId(int id) {
this.id = id;
}

@Column(name="sname", length=100,nullable=false)
private String sname;

/**
* @return the sname
*/
public String getSname() {
return sname;
}

/**
* @param sname the sname to set
*/
public void setSname(String sname) {
this.sname = sname;
}

@Column(name="sroll",nullable=false)
private int sroll;

/**
* @return the sroll
*/
public int getSroll() {
return sroll;
}

/**
* @param sroll the sroll to set
*/
public void setSroll(int sroll) {
this.sroll = sroll;
}

@Column(name="scourse",length=10,nullable=false)
private String scourse;

/**
* @return the scourse
*/
public String getScourse() {
return scourse;
}

/**
* @param scourse the scourse to set
*/
public void setScourse(String scourse) {
this.scourse = scourse;
}

}

Main Class: JPAGrouping.java

/**
*
*/
package roseindia;

import java.util.Iterator;
import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.Query;

/**
* @author Administrator
*
*/
public class JPAGrouping {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

EntityManagerFactory emf=Persistence.createEntityManagerFactory("jpa");
EntityManager em=emf.createEntityManager();
try{
EntityTransaction entr=em.getTransaction();
entr.begin();
Query query=em.createQuery("SELECT st FROM Student st group by st.sname");
List stList=query.getResultList();
Iterator stIterator=stList.iterator();
while(stIterator.hasNext()){
Student st=(Student)stIterator.next();
System.out.print("id:"+st.getId());
System.out.print(" sname:"+st.getSname());
System.out.print(" sroll:"+st.getSroll());
System.out.print(" scourse:"+st.getScourse());
System.out.println();
}
entr.commit();
}
finally{
em.close();
}

}

}

Output:

log4j:WARN No appenders could be found for logger
(org.hibernate.cfg.annotations.Version).

log4j:WARN Please initialize the log4j system properly.

Hibernate: select student0_.id as id0_, student0_.scourse as
scourse0_, student0_.sname as sname0_, student0_.sroll as
sroll0_ from student student0_ group by student0_.sname

id:2 sname:Ravi sroll:102 scourse:M.Tech.

id:3 sname:Suman Saurabh sroll:103 scourse:PHD

id:1 sname:Vinod sroll:101 scourse:MCA

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